The purpose of a catalytic converter for an internal combustion engine or a gas turbine is to convert pollutant materials in the exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxide, etc., to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square or triangular straight-through openings or cells with catalyst deposited on the walls of the cells; catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads; or a corrugated thin metal foil honeycomb monolith, e.g., a ferritic stainless steel foil or a nickel alloy, having a catalyst carried on or supported on the surface. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. Zeolite coatings may also be used for adsorption and desorption of the pollutants to aid in catalytic activity. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere.
However, conversion to such harmless by-products is not efficient initially when the exhaust gases are relatively cold, e.g., at cold start. To be effective at a high conversion rate, the catalyst and the surface of the converter with which the gases come in contact must be at or above a minimum temperature, e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC) and 1000 F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has reached its normal operating temperature, the catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases to make contact with a hot catalyst so as to effect satisfactory conversion. Compression ignited engines, spark ignited engines and reactors in gas turbines have this need.
To achieve initial heating of the catalyst at or prior to engine start-up, there is conveniently provided an electrically heatable catalytic converter, preferably one formed of a thin metal honeycomb monolith, either spaced flat thin metal strips, straight corrugated thin metal strips, pattern corrugated thin metal strips, (e.g., herringbone or chevron corrugated) or variable pitch corrugated thin metal strips (See U.S. Pat. No. 4,810,588 dated Mar. 7, 1989 to Bullock et al), or a combination thereof, which monolith is connected to a voltage source, e.g., a 12 volt to 108 volt DC power supply, preferably at the time of engine start-up and afterwards to elevate and maintain the catalyst to at least 650 F. plus or minus 30 F. Alternatively, power may also be supplied for a few seconds prior to start-up of the engine. Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since at least the early 1970's. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated Oct. 30, 1973. More recently, corrugated thin metal monoliths have been disclosed in U.S. Pat. No. 4,711,009 dated Dec. 8, 1987; U.S. Pat. No. 4,381,590 to Nonnenmann et al dated May 3, 1983, copending application U.S. Pat. Ser. No. 606,130 filed Oct. 31, 1990 by William A. Whittenberger and entitled Electrically Heatable Catalytic Converter and commonly owned with the present application now U.S. Pat. No. 5,070,694, and International PCT Publication Numbers WO 89/10470 (EP 412,086) and WO 89/10471 (EP 412,103) each filed Nov. 2, 1989, claiming a priority date of Apr. 25, 1988. The first two of the above International Publication Numbers disclose methods and apparatus for increasing the internal resistance by placing a series of spaced discs in series or electrically insulating intermediate layers. Another International PCT Publication Number is WO 90/12951 published Apr. 9, 1990 and claiming a priority date of Apr. 21, 1989 which seeks to improve axial strength by formlocking layers of insulated plates. Another reference which seeks to improve axial strength is the U.S. Pat. No. 5,055,275 dated Oct. 8, 1991 to Kannianen et al. However, a common problem with such prior devices has been their inability to survive severe automotive industry durability tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake Test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at high temperature (between 800 and 950 C.; 1472 to 1742 F., respectively) with exhaust gas from a running internal combustion engine simultaneously being passed through the device. If the catalytic device telescopes or displays separation or folding over of the leading or upstream edges of the foil leaves up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Usually, a device that lasts 5 hours will last 200 hours. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
The Hot Cycling test is conducted at 800 to 950 C. (1472 to 1742 F.) and cycled to 120 to 150 C. once every 15 to 20 minutes, for 300 hours. Telescoping or separation of the leading edges of the foil strips is considered a failure.
The Hot Shake Test and the Hot Cycling Test, hereinafter called "Hot Tests", have proved very difficult to survive, and many efforts to provide a successful device have been either too costly or ineffective for a variety of reasons.
Previously tested samples of EHC's in automotive service and comprised entirely of heater strips in electrical parallel did not have adequate endurance in Hot Tests nor did they have sufficiently high resistance to fulfill the need for lower power ratings. In repeated efforts to arrive at a suitable design using purely parallel circuit construction, samples were made and tested with a wide range of parameters, including a length-to-diameter aspect ratio of from 0.5 to 1.5, cell densities of from 100 to 500 cells per square inch, individual strip heaters as long as 20 inches, and parallel circuits limited to as few as 2 to 4 heater strips.
Devices made according to these design parameters proved unsatisfactory in the Hot Tests because (a) terminal resistance was too low and, therefore, the devices drew too much power, (b) the relatively high voltage differential between laminations associated with small numbers of parallel heater strips caused some interlaminar arcing, and (c) Hot Tests could not be passed consistently. With regard to (c), EHC's with heater strips longer than about 7" have generally not passed the Hot Shake Test. Resistance that is too low caused one or more of the following problems: (a) the battery becomes unacceptably large and expensive; (b) the EHC has to be made with longer heater strips which have a tendency to fail the Hot Tests.
Prior structures, such as that described in U.S. Pat. No. 4,928,485 have had all of the corrugated thin metal heater strip members connected in a manner such that all of the strips extended spirally outwardly from a central electrode to a circular shell which served as the electrode of opposite polarity. The strips serve as heaters for the core. For automotive purposes, terminal resistance must be of sufficient magnitude to limit the power to 2.0 KW or less at a terminal voltage of 7.0 volts. This power level cannot be achieved conveniently when all of the heater strips are of a desirable length for such construction, e.g., about 6.6", and connected in parallel.
It has now been found that the internal resistance of the core can be increased substantially by increasing the length of the individual core elements hereinafter called "heater strips," without increasing the honeycomb core diameter, by folding groups of heater strips over centrally located rigid posts and then spirally winding the assembly with insulation tape on either side of each group. The heater strips of the present invention must be nonnesting or spaced apart to permit the flow of exhaust gas over spaced catalyst bearing surfaces. Thus, the heater strips may be flat strips spaced, for example, as described in U.S. Pat. No. 4,942,020 dated Jul. 17, 1990 to Cornelison et al, or corrugated so as to be nonnesting, e.g., in a herringbone pattern. The middle portion of the heater strips must be flat. The free ends of the core elements, or heater strips, are connected to a segmented retaining shell, one segment or connector plate, being attached to one side of a voltage source, and another segment or connector plate being attached to the other side of the voltage source. The retainer shell segments are connected in series through the heater strips. In preferred embodiments, there are two folded over groups of strips, each group bent around a centrally located rigid central post. A U-shaped pin, not unlike a cotter pin, may be used for this purpose. Each "group" may comprise a single corrugated thin high temperature resistive metal alloy heater strip, say 13 inches long, or 2, 3, 4, or more such strips in laminar relation. The strips are preferably corrugated in such a manner as to be nonnesting, or the strips may be arranged with a corrugated strip alternating with a flat strip of substantially equal length to avoid nesting. One end of each member of a group of thin high temperature resistive metal alloy heater strips is electrically secured to one retaining shell segment, and the other end of each member of a group of thin high temperature resistive metal alloy heater strips is electrically secured to another retaining shell segment.
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for ferritic stainless steel alloy is described in U.S. Pat. No. 4,414,023 to Aggen data Nov. 8, 1983. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. Another metal especially useful herein is identified as Haynes 214 alloy which is commercially available. This alloy is described in U.S. Pat. No. 4,671,931 dated Jun. 9, 1987 to Herchenroeder et al. This alloy is characterized by high resistance to oxidation. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more Rare Earth metals except yttrium, 0.05% carbon, and steel making impurities. Ferritic stainless steel (commercially available as Alfa IV from Allegheny Ludlum Steel Co.) and Haynes 214 alloy are examples of high temperature resistive, oxidation resistant metals that are suitable for use in making heater strips for EHC cores hereof. Suitable metals must be able to withstand temperatures of 900 C. to 1100 C. over prolonged periods.
Other high temperature resistive, oxidation resistant metals are known and may be used herein. For automative applications, for example, the thickness of the metal foil heater strips is in the range of from 0.0015" to 0.003", preferably 0.0016" to 0.002".
In the following description, reference will also be made to fibrous ceramic mat, woven fabrics, or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Sowman and to the U.S. patent to Hatch U.S. Pat. No. 3,916,057 dated Oct. 28, 1975 for formulations and manufacture of ceramic fiber tapes and mats useful herein. One such woven ceramic fiber material is currently available from 3-M under the registered trademark "NEXTEL" 312 Woven Tape useful for isolating the respective groups of strips as described below. Ceramic fiber mat is available as "INTERAM" also from 3-M.